1. Field of the Invention
The present invention relates generally to a cathode-ray tube, and more particularly to a color cathode-ray tube including a grid structure that can stabilize, in a short time from the start of operation, a cathode current value of an in-line electron gun assembly that is mounted in the cathode-ray tube.
2. Description of the Related Art
In a color cathode-ray tube, which is currently used in general, an in-line electron gun assembly that emits three electron beams for blue, green and red, which are horizontally arranged in line, is provided within the neck of the envelope. Electron beams that are emitted from the electron gun assembly are deflected by polarizing magnetic fields that are generated by a deflection yoke. Thereby, the electron beams are caused to scan the phosphor screen in the horizontal and vertical directions. Thus, a color image is reproduced on the phosphor screen.
The above-described in-line electron gun assembly greatly contributes to enhancement in quality and performance of the color cathode-ray tube, and it is widely employed in modern cathode-ray tubes. The electron gun assembly includes three cathodes, which are horizontally arranged in line, three heaters, which are inserted in the respective cathodes, and a plurality of grids, which are coaxially arranged at predetermined intervals from the cathode side toward the phosphor screen.
As is shown in FIG. 7 and FIG. 8, for example, an electron beam generating section that generates electron beams toward the phosphor screen includes cylindrical cathodes K (KB, KG, KR), cup-shaped first grids G1 that control the electron beams emitted from the cathodes K, and a plate-shaped second grid G2 that accelerates the electron beams. The cathodes K, first grid G1 and second grid G2 are coaxially arranged at predetermined intervals and are fixed by an insulation glass 66. Three cathode support members 63 hold the associated cathodes K. An insulation member 64 integrally holds the cathode support members 63. A metal cylinder 65 surrounds the outer periphery of the insulation member 64.
In the color cathode-ray tube using the above-described in-line electron gun assembly 57, in order to obtain a good white screen, cutoff voltages in the electron beam generating section that generates the respective three electron beams, are designed to become equal. In other words, each of cathode current values (IK values) of the respective cathodes K is designed to be set at a predetermined fixed value.
In general, the respective cutoff voltages in the electron gun assembly 57 do not become equal due to non-uniformity in components or a difference in thermal expansion coefficient. In order to set the IK value of each cathode K at a predetermined fixed value, bias voltages are adjusted in accordance with the characteristics of the respective cathodes K after the color cathode-ray tube is assembled in a cathode-ray tube apparatus such as a TV receiver. Thereby, equality between IK values is ensured.
Even if such a measure is taken to secure equality between IK values, the IK values of the cathodes cannot be set at a predetermined fixed value during a warming-up time beginning with turn-on of the heaters and continuing until thermal deformation of each component reaches an equilibrium state. In other words, even if the bias voltages are adjusted to secure the equality between IK values, the equality between IK values is actually obtained after the end of the warming-up time. The time needed for the warming-up is about 20 minutes after power-on.
To be more specific, when the heaters are turned on and the respective components reach the thermal equilibrium state, the cathodes K operate in a higher temperature range than the cathode support members 63, and the cathode support members 63 operate in a higher temperature range than the insulation member 64. In other words, the rate of temperature rise is higher in the cathode support members 63 than in the insulation member 64 that supports the cathode support members 63, and the rate of temperature rise is higher in the cathodes K than in the cathode support members 63.
As a result, the respective electrode components of the electron gun assembly 57 have different thermal deformation characteristics because of thermal expansion due to different rates of temperature rise. The thermal deformation is explained referring to FIG. 9 and FIG. 10. In FIG. 9, arrows indicate directions of extension of associated structural components. In FIG. 10, characteristic curves A, B and C correspond to arrows A, B and C in FIG. 9.
As is indicated by arrow A in FIG. 9, the cathode K extends toward the first grid G1 due to temperature rise during the warming-up time. The shape of the cathode K varies with time, as indicated by characteristic curve A in FIG. 10. By contrast, as is indicated by arrow B in FIG. 9, the cathode support member 63 extends in a direction away from the first grid G1, that is, toward the stem side, due to temperature rise during the warming-up time. The shape of the cathode support member 63 varies with time, as indicated by characteristic curve B in FIG. 10. In addition, as indicated by arrow C in FIG. 9, the first grid G1 extends in a direction away from the cathode K due to temperature rise, and the shape thereof varies as indicated by characteristic curve C in FIG. 10.
The cathode K and cathode support member 63 are disposed close to the heater and are formed using thin plates. Thus, the cathode K reaches a thermal equilibrium state within about 30 seconds from power-on, and the cathode support member 63 reaches a thermal equilibrium state in a relatively short time period of about three minutes.
By contrast, the first grid G1 reaches a thermal equilibrium state about 15 minutes after power-on. Thus, after the passage of about three minutes from the beginning of the warming-up time period, the distance between the cathode K and first grid G1 varies mainly due to thermal deformation of the first grid G1.
Consequently, in the warming-up time period, the distance between the cathode K and first grid G1 varies, as shown by curve D in FIG. 10, due to the above-mentioned thermal expansion of the respective components. Since the distance between the cathode K and first grid G1 varies in the warming-up time period, about 15 minutes from power-on of the heater are needed until the respective components reach an equilibrium state of operational temperatures.
In addition, after the end of warming-up, in order to keep equal the IK values of the three cathodes K, bias voltages are adjusted so as to correct a non-uniform distance between the electrodes, in particular, between the cathode K and first grid G1. However, after the color cathode-ray tube is assembled in the cathode-ray tube apparatus, the once set bias voltages cannot be adjusted unless the cathode-ray tube apparatus is opened. It is difficult for the user to perform such adjustment. Disadvantageously, the once set equality between IK values cannot actually be achieved until the operation temperature reaches the thermal equilibrium state.
FIG. 11 shows examples of IK value curves that indicate variations with time from power-on until each IK value reaches a predetermined fixed value. In these examples, bias voltages are adjusted such that the cathode current values of the center beam cathode KG, which is disposed substantially at the center of the neck, and the side beam cathodes KR and KB, which are disposed on both sides of the cathode KG, may become 10 μA after the end of warming-up. Thereafter, sufficiently cooling is conducted in the state in which the heater voltage alone is turned off. Then, the heater is powered on. FIG. 11 shows resultant variations with time of cathode current values. In FIG. 11, curve I indicates IK value characteristics of the center beam cathode KG of the three in-line cathodes K. Curve II indicates IK value characteristics of the side beam cathode KB (or KR).
The IK value characteristics of the curves I and II are different because the first grid G1G for the center beam and the first grids G1B and G1R for the side beams are differently affected by the temperatures and thermal expansion of the electrode components that are disposed on the periphery of each first grid G1.
In FIG. 11, as regards both curves I and II, a variation in IK value within about 30 seconds after power-on of the heater is mainly due to the fact that the cathode K extends toward the first grid G1. In addition, a variation in IK value in a time period between about 30 seconds and about three minutes after power-on of the heater is mainly due to the fact that the cathode support member 63 extends in a direction away from the first grid G1. A variation in IK value after the passage of about three minutes from power-on of the heater is mainly due to the thermal deformation of the first grid G1.
In particular, as regards the in-line electron gun assembly 57 including the individual first grids G1 (G1B, G1G, G1R), as shown in FIG. 12, implant parts for fixing the three first grids G1B, G1G and G1R to the insulation glass 66 need to be configured such that the three first grids G1 (G1B, G1G, G1R) are accommodated within the limited inside diameter of the neck and are completely supported and held within the width of the insulation glass 66 in the in-line direction. To meet this requirement, an implant member 68 for the first grid G1G (center beam) and implant members 69 for the first grids G1B and G1R (both side beams) need to have different shapes.
Specifically, the implant member 68 for first grid G1G (center beam) is formed to be axisymmetric with respect to both an in-line directional axis 70 of beam passage holes and a vertical axis 71 that intersects at right angles with the in-line directional axis 70 and passes through the center beam passage hole. By contrast, each of the implant members 69 for first grids G1B and G1R (both side beams) is formed to be axisymmetric with respect to the in-line directional axis 70 and to be non-axisymmetric with respect to a vertical axis 72 that intersects at right angles with the in-line directional axis 70 and passes through the associated side beam passage hole.
Thereby, the first grid G1G and the two first grids G1B and G1R can have substantially equal temperatures when they reach the thermally stable state. However, the first grid G1G and two first grids G1B and G1R require different times until they reach the thermally stable state. To be more specific, the first grid G1G has both sides and vertical ends surrounded by the other electrodes and insulation glass. On the other hand, only the first grid G1G is disposed around each of the first grid G1B and G1R. Consequently, the first grid G1G requires a longer time than each of the first grids G1B and G1R before reaching the thermally stable state.
As stated above, the first grid G1G and each of the first grids G1B and G1R reach the set IK value in different modes. Hence, a time period that is approximately equal to the warming-up time is needed until the equality of IK values of the three cathodes K is attained and a good white screen is obtained.
Consequently, a considerable length of time is required until a good white screen is obtained after power-on of the color cathode-ray tube. Moreover, it is difficult for a general user to adjust bias voltages after the color cathode-ray tube is assembled in the cathode-ray tube apparatus. Thus, the equality between IK values that are once set cannot actually be obtained until the respective components reach the equilibrium state of operational temperatures. A long time is needed in order to obtain a good white screen. This means that a low-quality image is displayed for a long time after power-on of the cathode-ray tube apparatus.
To solve the above-problem, there is prior art (e.g. Jpn. Pat. Appln. KOKAI Publication No. 2002-184320) wherein the relationship between the length to a fixing point between a cathode support member for a center beam and a metal cylinder disposed around this cathode support member, on the one hand, and the length to a fixing point between a cathode support member for a side beam and a metal cylinder disposed around this cathode support member, on the other hand, is defined with reference to a fixing point between each cathode and the associated cathode support member. Further, there is prior art (e.g. Jpn. Pat. Appln. KOKAI Publication No. 2002-110056) wherein the relationship between the length from the fixing point between a center beam cathode and an associated cathode support member to an insulation member, on the one hand, and the length from the fixing point between a side beam cathode and an associated support member to the insulation member, on the other hand, is defined.